As wireless technology has advanced, a variety of wireless networks have been installed, such as cellular and other wireless networks. Some wireless networks are based upon the Institute of Electrical and Electronics Engineers (IEEE) 802.11 family of Wireless LAN (WLAN) industry specifications, for example. As another example, some wireless networks are based upon the Distributed Medium Access Control (MAC) for Wireless Networks industry specifications of the WiMedia Alliance, for example. For example, the WiMedia network protocol adaptation (WiNet) layer is a protocol adaptation layer (PAL) that builds on a WiMedia ultra-wideband (UWB) common radio platform to augment the convergence platform with TCP/IP services. A number of working groups are working to improve on this technology.
An example standard, for example, the Distributed Medium Access Control (MAC) for Wireless Networks of the WiMedia Alliance, defines a distributed medium access control (MAC) sublayer for wireless networks, and further specifies a wireless network structure that does not require an existing infrastructure for communication such as, for example, a WiMedia ultra-wideband (UWB) network.
Categories of example applications considered for such an example standard may include portable electronic devices intended to be carried by a user, home electronics equipment, and personal computers and peripherals. Example portable electronic devices may have specific requirements to support mobility and good power efficiency. Devices such as home electronics and computers may not be as mobile, and not as sensitive to power efficiency as such portable electronic devices. All of these devices may benefit from a zero-infrastructure environment.
An interval, for example, a periodic time interval may be used to coordinate frame transmissions between devices, for example, a superframe interval may be used which includes a beacon period followed by a data period. The beacon period may include multiple beacon slots which may be used by multiple devices to send beacons.
In an example network formed with fully distributed medium access coordination, logical groups may be formed around each device in the network to facilitate contention-free frame exchanges while exploring medium reuse over different spatial regions. These logical groups may include, for example, a beacon group and an extended beacon group, both of which may be determined with respect to an individual device. For example, a beacon group may include a set of devices from which a device receives beacons that identify the same beacon period start time (BPST) as the device. An extended beacon group may include a union of a device's beacon group and the beacon groups of all devices in the device's beacon group.
Example MAC protocol algorithms may attempt to ensure that no member of an extended beacon group transmits a beacon frame at the same time as the device. Information included in beacon frames may facilitate contention-free frame exchanges by ensuring that a device does not transmit frames while a neighbor of the device (e.g., another device in the device's beacon group) is transmitting or receiving frames.
When a device is enabled, it may scan one or more channels for beacons and select a communications channel. If no beacons are detected in the selected channel, the device may create its own beacon period (BP) by sending a beacon. If one or more beacons are detected in the selected channel, the device may synchronize its BP to existing beacons in the selected channel. The device may then exchange data with members of its beacon group using the same channel the device selected for beacons.
Each device may protect its and its neighbors' BPs for exclusive use of the beacon protocol. Thus, no transmissions other than beacons may be attempted during the BP of any device. A device may protect an alien BP, detected by reception of a beacon frame unaligned with the device's own BP, by announcing a reservation covering the alien BP in its beacon. Within the context of a particular beacon group, an alien beacon group may include one or more devices included in a beacon group that identify a beacon period start time (BPST) that is different from the particular beacon group.
An example WiMedia standard also defines a dynamic beaconing technique, which enables devices in a distributed network to maintain fast connectivity. Devices may maintain synchronization with each other by participating in a beacon period, for example, by each device sending its own beacon and listening to other devices' beacons once in each superframe (e.g., 65.536 ms). The rest of the time the devices may send data to each other or hibernate, or sleep.
If a group of devices moves into the range of another group of devices, the groups may need to synchronize to each other before connectivity from one group to another may be available for the devices, and before channel time reservations may be handled without collisions. A group of devices may thus be viewed as “one device” or “two or more devices participating in the same beacon group,” for example, devices having the same beacon period start time (BPST).
Establishing synchronization between the groups may involve regular scanning activities. Scanning may be performed at a device by listening to the channel, for example, for at least the time associated with one superframe occasionally. The scanning may be repeated based on an expectation of the connectivity speed. For example, if one superframe time is scanned once every second, the new devices or groups of devices on the same communications channel may be found on average in half a second when they enter the operating range. Scanning may also be subdivided into shorter pieces, but the amount of the total scanning time may be significant for finding the new devices within the range.
The power consumption caused by regular scanning may be significant, especially for a battery-powered device. For example, a device may participate in a beacon period (e.g., for an activity time of 0.5 ms for a small number of devices in the beacon group) every superframe and may perform a superframe scan once every two seconds, for example, with no data exchange. The power consumption attributed to the scanning for this example is four times the power consumption attributed to the beaconing. Also, if the device hibernates, participating in only a small part of the beacon periods, the scanning may still be required, but the scanning operation may consume an even higher percentage of the whole power consumption of the device.